The invention relates generally to phased-locked loop demodulators, and more particularly to a complex digital phase-locked loop demodulator.
Phase-locked loop demodulators that are capable of demodulating low IF signals are presently available. These devices convert the RF carrier to the I and Q quadrature components at low IF which are processed in a phase-locked loop. An example of such a demodulator is described in U.S. Pat. No. 4,852,123 which issued on Jul. 25, 1989 to Bickley et al.
These demodulators are generally unstable as the IF nears zero-IF. In addition, these circuits which include loop filters as well as a voltage controlled oscillator with further filters and mixers to drive the phase-locked loop, are quite complicated.
Therefore, there is a need for a simpler coherent FM demodulator in low-IF or zero-IF for radio receiver architectures. In addition, it is desirable to coherently demodulate FM signals with extremely low modulation indexes and significant centre frequency offset even when such offsets are several times greater than the maximum FM modulation deviation.
The invention is directed to a complex demodulator for demodulating a received RF signal converted to digital I and Q components of the IF signal comprising a numerically controlled oscillator (NCO), four digital multipliers each having a first input, a second input and an output, a difference combiner and a summing combiner that are formed into a complex phase locked loop. The numerically controlled oscillator (NCO) generate first and second output signals that are in quadrature and centred at the IF frequency. The digital I IF signal is coupled to the first input of each of the first and third multipliers and the digital Q IF signal is coupled to the first input of each of the second and fourth multipliers. The first NCO output signal is coupled to the second input of the first and fourth multipliers while the second NCO output signal is coupled second input of the second and third multipliers. The outputs of the first and second multipliers are coupled to the difference combiner to provide a phase output while the outputs of the third and fourth multipliers are coupled to the summing combiner to provide an amplitude output. The phase output is also coupled through a feedback loop to the NCO.
In accordance with another aspect of the invention, the complex demodulator may further include a loop filter in the feedback loop between the output of the difference combiner and the NCO. The filter may be a single pole filter to produce a second order loop or have two or more poles to produce a third or higher order loop.
Regarding another aspect of the invention, the NCO first output signal may be a sine signal and the NCO second output signal may be cos signal.
Other aspects and advantages of the invention, as well as the structure and operation of various embodiments of the invention, will become apparent to those ordinarily skilled in the art upon review of the following description of the invention in conjunction with the accompanying drawings.